Heterosis in F2-Generations of Drosophila-Melanogaster

Abstract

Three crosses among inbred lines of Drosophila melanogaster generated by eight generation full-sib mating, three Australasian population crosses, and two Australian/English population crosses were raised to the F2 generation, to examine evidence of hybrid breakdown. Inbred line crosses produced F2 generations that retained, on average, 54% of the F1 heterosis, which was not significantly different from the retention of 50% F1 heterosis expected under a dominance model of heterosis. Australasian population crosses produced F2 generations with significantly less than 50% of the F1 heterosis, while crosses between Australian and English populations produced a F2 generation that generally performed worse than midparent purebred populations. These results imply epistatic gene action. Because inbred lines contain random combinations of genes, recombination does not necessarily produce hybrid breakdown. However, in crosses between natural populations, recombination is likely to disrupt co-adapted gene combinations leading to poor F2 performance. Populations which rarely exchange migrants are more likely to have alternative gene combinations and to show hybrid breakdown than populations which regularly exchange migrants. The implications of these findings for livestock crossbreeding programs is discussed.